Dual magnetic tunnel junction sensor with a longitudinal bias stack

ABSTRACT

A dual magnetic tunnel junction (MTJ) sensor is provided with a longitudinal bias stack sandwiched between a first MTJ stack and a second MTJ stack. The longitudinal bias stack comprises an antiferromagnetic (AFM) layer sandwiched between first and second ferromagnetic layers. The first and second MTJ stacks comprise antiparallel (AP)-pinned layers pinned by AFM layers made of an AFM material having a higher blocking temperature than the AFM material of the bias stack allowing the AP-pinned layers to be pinned in a transverse direction and the bias stack to be pinned in a longitudinal direction. The demagnetizing fields of the two AP-pinned layers cancel each other and the bias stack provides flux closures for the sense layers of the first and second MTJ stacks.

CROSS REFERENCE TO RELATED APPLICATION

[0001] U.S. patent application docket number SJO9-2000-00163US1,entitled DUAL SPIN VALVE SENSOR WITH A LONGITUDINAL BIAS STACK, wasfiled on the same day, owned by a common assignee and having the sameinventors as the present invention.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates in general to magnetic transducers forreading information signals from a magnetic medium and, in particular,to a dual magnetic tunnel junction sensor with a longitudinal bias stackbetween first and second magnetic tunnel junction structures of the dualsensor.

[0004] 2. Description of the Related Art

[0005] Computers often include auxiliary memory storage devices havingmedia on which data can be written and from which data can be read forlater use. A direct access storage device (disk drive) incorporatingrotating magnetic disks is commonly used for storing data in magneticform on the disk surfaces. Data is recorded on concentric, radiallyspaced tracks on the disk surfaces. Magnetic heads including readsensors are then used to read data from the tracks on the disk surfaces.

[0006] In high capacity disk drives, magnetoresistive (MR) read sensors,commonly referred to as MR sensors, are the prevailing read sensorsbecause of their capability to read data from a surface of a disk atgreater track and linear densities than thin film inductive heads. An MRsensor detects a magnetic field through the change in the resistance ofits MR sensing layer (also referred to as an “MR element”) as a functionof the strength and direction of the magnetic flux being sensed by theMR layer.

[0007] The conventional MR sensor operates on the basis of theanisotropic magnetoresistive (AMR) effect in which an MR elementresistance varies as the square of the cosine of the angle between themagnetization in the MR element and the direction of sense currentflowing through the MR element. Recorded data can be read from amagnetic medium because the external magnetic field from the recordedmagnetic medium (the signal field) causes a change in the direction ofmagnetization in the MR element, which in turn causes a change inresistance in the MR element and a corresponding change in the sensedcurrent or voltage.

[0008] Another type of MR sensor is the giant magnetoresistance (GMR)sensor manifesting the GMR effect. In GMR sensors, the resistance of theMR sensing layer varies as a function of the spin-dependent transmissionof the conduction electrons between magnetic layers separated by anonmagnetic spacer layer and the accompanying spin-dependent scatteringwhich takes place at the interface of the magnetic and nonmagneticlayers and within the magnetic layers.

[0009] GMR sensors using only two layers of ferromagnetic material(e.g., Ni—Fe) separated by a layer of nonmagnetic material (e.g.,copper) are generally referred to as spin valve (SV) sensors manifestingthe SV effect.

[0010]FIG. 1 shows a prior art SV sensor 100 comprising end regions 104and 106 separated by a central region 102. A first ferromagnetic layer,referred to as a pinned (or reference) layer 120, has its magnetizationtypically fixed (pinned) by exchange coupling with an antiferromagnetic(AFM) layer 125. The magnetization of a second ferromagnetic layer,referred to as a free (or sense) layer 110, is not fixed and is free torotate in response to the magnetic field from the recorded magneticmedium (the signal field). The free layer 110 is separated from thepinned layer 120 by a nonmagnetic, electrically conducting spacer layer115. Hard bias layers 130 and 135 formed in the end regions 104 and 106,respectively, provide longitudinal bias for the free layer 110. Leads140 and 145 formed on hard bias layers 130 and 135, respectively,provide electrical connections for sensing the resistance of SV sensor100. In the SV sensor 100, because the sense current flow between theleads 140 and 145 is in the plane of the SV sensor layers, the sensor isknown as a current-in-plane (CIP) SV sensor. IBM's U.S. Pat. No.5,206,590 granted to Dieny et al., incorporated herein by reference,discloses a SV sensor operating on the basis of the GMR effect.

[0011] Another type of spin valve sensor is an antiparallel (AP)-pinnedSV sensor. The AP-pinned SV sensor differs from the simple spin valvesensor in that an AP-pinned structure has multiple thin film layersinstead of a single pinned layer. The AP-pinned structure has anantiparallel coupling (APC) layer sandwiched between first and secondferromagnetic pinned layers. The first pinned layer has itsmagnetization oriented in a first direction by exchange coupling to theantiferromagnetic (AFM) pinning layer. The second pinned layer isimmediately adjacent to the free layer and is antiparallel exchangecoupled to the first pinned layer because of the minimal thickness (inthe order of 8 Å) of the APC layer between the first and second pinnedlayers. Accordingly, the magnetization of the second pinned layer isoriented in a second direction that is antiparallel to the direction ofthe magnetization of the first pinned layer.

[0012] The AP-pinned structure is preferred over the single pinned layerbecause the magnetizations of the first and second pinned layers of theAP-pinned structure subtractively combine to provide a net magnetizationthat is much less than the magnetization of the single pinned layer. Thedirection of the net magnetization is determined by the thicker of thefirst and second pinned layers. A reduced net magnetization equates to areduced demagnetization field from the AP-pinned structure. Since theantiferromagnetic exchange coupling is inversely proportional to the netmagnetization, this increases exchange coupling between the first pinnedlayer and the antiferromagnetic pinning layer. The AP-pinned spin valvesensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heimand Parkin which is incorporated by reference herein.

[0013] Another type of magnetic device currently under development is amagnetic tunnel junction (MTJ) device. The MTJ device has potentialapplications as a memory cell and as a magnetic field sensor. The MTJdevice comprises two ferromagnetic layers separated by a thin,electrically insulating, tunnel barrier layer. The tunnel barrier layeris sufficiently thin that quantum-mechanical tunneling of chargecarriers occurs between the ferromagnetic layers. The tunneling processis electron spin dependent, which means that the tunneling currentacross the junction depends on the spin-dependent electronic propertiesof the ferromagnetic materials and is a function of the relativeorientation of the magnetizations of the two ferromagnetic layers. Inthe MTJ sensor, one ferromagnetic layer has its magnetization fixed, orpinned, and the other ferromagnetic layer has its magnetization free torotate in response to an external magnetic field from the recordingmedium (the signal field). When an electric potential is applied betweenthe two ferromagnetic layers, the sensor resistance is a function of thetunneling current across the insulating layer between the ferromagneticlayers. Since the tunneling current that flows perpendicularly throughthe tunnel barrier layer depends on the relative magnetizationdirections of the two ferromagnetic layers, recorded data can be readfrom a magnetic medium because the signal field causes a change ofdirection of magnetization of the free layer, which in turn causes achange in resistance of the MTJ sensor and a corresponding change in thesensed current or voltage. IBM's U.S. Pat. No. 5,650,958 granted toGallagher et al., incorporated in its entirety herein by reference,discloses an MTJ sensor operating on the basis of the magnetic tunneljunction effect.

[0014]FIG. 2 shows a prior art MTJ sensor 200 comprising a firstelectrode 204, a second electrode 202, and a tunnel barrier layer 215.The first electrode 204 comprises a pinned layer (pinned ferromagneticlayer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240.The magnetization of the pinned layer 220 is fixed through exchangecoupling with the AFM layer 230. The second electrode 202 comprises afree layer (free ferromagnetic layer) 210 and a cap layer 205. The freelayer 210 is separated from the pinned layer 220 by a nonmagnetic,electrically insulating tunnel barrier layer 215. In the absence of anexternal magnetic field, the free layer 210 has its magnetizationoriented in the direction shown by arrow 212, that is, generallyperpendicular to the magnetization direction of the pinned layer 220shown by arrow 222 (tail of an arrow pointing into the plane of thepaper). A first lead 260 and a second lead 265 formed in contact withfirst electrode 204 and second electrode 202, respectively, provideelectrical connections for the flow of sensing current Is from a currentsource 270 to the MTJ sensor 200. Because the sensing current isperpendicular to the plane of the sensor layers, the MTJ sensor 200 isknown as a current-perpendicular-to-plane (CPP) sensor. A signaldetector 280, typically including a recording channel such as apartial-response maximum-likelihood (PRML) channel, connected to thefirst and second leads 260 and 265 senses the change in resistance dueto changes induced in the free layer 210 by the external magnetic field.

[0015] Two types of current-perpendicular-to-plane (CPP) sensors havebeen extensively explored for magnetic recording at ultrahigh densities(≧20 Gb/in²). One is a GMR spin valve sensor and the other is a MTJsensor. Two challenging issues are encountered when the CPP sensor isused for ever increasing magnetic recording densities. First, the GMRcoefficient may not be high enough to ensure adequate signal amplitudeas the sensor width is decreased and second, magnetic stabilization ofthe sense layer can be difficult due to the use of insulating layers toavoid current shorting around the active region of the sensor. A dualCPP sensor can be used to provide increased magnetoresistive response toa signal field due to the additive response of the dual sensors. IBM'sU.S. Pat. No. 5,287,238 granted to Baumgart et al. discloses a dual CIPSV sensor. However, sensor stability still remains a major concern.

[0016] There is a continuing need to increase the MR coefficient andreduce the thickness of GMR sensors while improving sensor stability. Anincrease in the GMR coefficient and reduced sensor geometry equates tohigher bit density (bits/square inch of the rotating magnetic disk) readby the read head.

SUMMARY OF THE INVENTION

[0017] It is an object of the present invention to disclose a dualmagnetic tunnel junction (MTJ) sensor with improved sensorstabilization.

[0018] It is another object of the present invention to disclose a dualMTJ sensor having a longitudinal bias stack between a first MTJ stackand a second MTJ stack to provide improved stabilization of the sense(free) layers of the first and second MTJ stacks.

[0019] It is a further object of the present invention to disclose adual MTJ sensor having a longitudinal bias stack comprising a firstdecoupling layer, a first ferromagnetic (FM1) layer, anantiferromagnetic (AFM) layer, a second ferromagnetic (FM2) layer and asecond decoupling layer disposed between the sense layers of first andsecond MTJ stacks.

[0020] It is yet another object of the present invention to disclose adual MTJ sensor having a longitudinal bias stack disposed between firstand second MTJ stacks to provide three flux closures for improved sensorstability. A first flux closure provides stability of the first MTJstack, a second flux closure provides stability of the second MTJ stack,and a third flux closure provides cancellation of demagnetizing fieldsfrom first and second antiparallel (AP)-pinned layers of the dual MTJsensor.

[0021] In accordance with the principles of the present invention, thereis disclosed a preferred embodiment of the present invention wherein adual MTJ sensor comprises a first MTJ stack, a second MTJ stack and alongitudinal bias stack disposed between first and second sense layersof the dual MTJ sensor. The first MTJ stack comprises a firstantiferromagnetic (AFM1) layer, a first AP-pinned layer, a first tunnelbarrier layer and a first sense layer. The second MTJ stack comprises asecond antiferromagnetic (AFM2) layer, a second AP-pinned layer, asecond tunnel barrier layer and a second sense layer. The longitudinalbias stack comprises a third antiferromagnetic (AFM3) layer sandwichedbetween a first ferromagnetic (FM1) layer and a second ferromagnetic(FM2) layer, and first and second decoupling layers in laminar contactwith the FM1 and FM2 layers, respectively.

[0022] The AFM1 and AFM2 layers are set by annealing the MTJ sensor atelevated temperature (about 280° C.) in a large magnetic field (about10,000 Oe) oriented in a transverse direction perpendicular to an airbearing surface (ABS) to orient the magnetizations of the first andsecond AP-pinned layers. The AFM3 layer, formed of antiferromagneticmaterial having a lower blocking temperature (temperature at which thepinnning field reaches zero Oe) than AFM1 and AFM2, is set by theannealing but is reset by a second annealing step at a lower temperature(about 240° C.) in a smaller magnetic field (about 200 Oe) oriented in alongitudinal direction parallel to the ABS to reorient themagnetizations of the FM1 and FM2 layers from the transverse to thelongitudinal direction without reorienting magnetizations of the firstand second AP-pinned layers. After the two annealing steps, themagnetizations of the first and second AP-pinned layers are orientedperpendicular to the ABS with net magnetic moments canceling each other,and the FM1 and FM2 layers are oriented in the longitudinal direction.The magnetization of the FM1 layer forms a flux closure with themagnetization of the first sense layer and the magnetization of the FM2layer forms a flux closure with the magnetization of the second senselayer. The first and second sense layers can be stabilized throughmagnetostatic interactions induced from the first and second fluxclosures, respectively.

[0023] The above as well as additional objects, features, and advantagesof the present invention will become apparent in the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] For a fuller understanding of the nature and advantages of thepresent invention, as well as the preferred mode of use, referenceshould be made to the following detailed description read in conjunctionwith the accompanying drawings. In the following drawings, likereference numerals designate like or similar parts throughout thedrawings.

[0025]FIG. 1 is an air bearing surface view, not to scale, of a priorart SV sensor;

[0026]FIG. 2 is an air bearing surface view, not to scale, of a priorart magnetic tunnel junction sensor;

[0027]FIG. 3 is a simplified diagram of a magnetic recording disk drivesystem using the MTJ sensor of the present invention;

[0028]FIG. 4 is a vertical cross-section view, not to scale, of a“piggyback” read/write head;

[0029]FIG. 5 is a vertical cross-section view, not to scale, of a“merged” read/write head; and

[0030]FIG. 6 is an air bearing surface view, not to scale, of apreferred embodiment of a dual MTJ sensor according to the presentinvention; and

[0031]FIG. 7 is an air bearing surface view, not to scale, of a anotherembodiment of a dual MTJ sensor according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] The following description is the best embodiment presentlycontemplated for carrying out the present invention. This description ismade for the purpose of illustrating the general principles of thepresent invention and is not meant to limit the inventive conceptsclaimed herein.

[0033] Referring now to FIG. 3, there is shown a disk drive 300embodying the present invention. As shown in FIG. 3, at least onerotatable magnetic disk 312 is supported on a spindle 314 and rotated bya disk drive motor 318. The magnetic recording media on each disk is inthe form of an annular pattern of concentric data tracks (not shown) onthe disk 312.

[0034] At least one slider 313 is positioned on the disk 312, eachslider 313 supporting one or more magnetic read/write heads 321 wherethe head 321 incorporates the dual MTJ sensor of the present invention.As the disks rotate, the slider 313 is moved radially in and out overthe disk surface 322 so that the heads 321 may access different portionsof the disk where desired data are recorded. Each slider 313 is attachedto an actuator arm 319 by means of a suspension 315. The suspension 315provides a slight spring force which biases the slider 313 against thedisk surface 322. Each actuator arm 319 is attached to an actuator 327.The actuator as shown in FIG. 3 may be a voice coil motor (VCM). The VCMcomprises a coil movable within a fixed magnetic field, the directionand speed of the coil movements being controlled by the motor currentsignals supplied by a controller 329.

[0035] During operation of the disk storage system, the rotation of thedisk 312 generates an air bearing between the slider 313 (the surface ofthe slider 313 which includes the head 321 and faces the surface of thedisk 312 is referred to as an air bearing surface (ABS)) and the disksurface 322 which exerts an upward force or lift on the slider. The airbearing thus counterbalances the slight spring force of the suspension315 and supports the slider 313 off and slightly above the disk surfaceby a small, substantially constant spacing during normal operation.

[0036] The various components of the disk storage system are controlledin operation by control signals generated by the control unit 329, suchas access control signals and internal clock signals. Typically, thecontrol unit 329 comprises logic control circuits, storage chips and amicroprocessor. The control unit 329 generates control signals tocontrol various system operations such as drive motor control signals online 323 and head position and seek control signals on line 328. Thecontrol signals on line 328 provide the desired current profiles tooptimally move and position the slider 313 to the desired data track onthe disk 312. Read and write signals are communicated to and from theread/write heads 321 by means of the recording channel 325. Recordingchannel 325 may be a partial response maximum likelihood (PMRL) channelor a peak detect channel. The design and implementation of both channelsare well known in the art and to persons skilled in the art. In thepreferred embodiment, the recording channel 325 is a PMRL channel.

[0037] The above description of a typical magnetic disk storage system,and the accompanying illustration of FIG. 3 are for representationpurposes only. It should be apparent that disk storage systems maycontain a large number of disks and actuator arms, and each actuator armmay support a number of sliders.

[0038]FIG. 4 is a side cross-sectional elevation view of a “piggyback”magnetic read/write head 400, which includes a write head portion 402and a read head portion 404, the read head portion employing a dual MTJsensor 406 according to the present invention. The MTJ sensor 406 issandwiched between ferromagnetic first and second shield layers 412 and414 at the ABS 440. A nonmagnetic insulating layer 409 is sandwichedbetween the first and second shield layers 412 and 414 in the regionbehind the sensor extending away from the ABS to prevent shortingbetween the shield layers. In response to external magnetic fields, theresistance of the SV sensor 406 changes. A sense current Is conductedthrough the sensor causes these resistance changes to be manifested asvoltage changes. These voltage changes are then processed as readbacksignals by the processing circuitry of the data recording channel 346shown in FIG. 3.

[0039] The write head portion 402 of the magnetic read/write head 400includes a coil layer 416 sandwiched between first and second insulationlayers 418 and 420. A third insulation layer 522 may be employed forplanarizing the head to eliminate ripples in the second insulation layer420 caused by the coil layer 416. The first, second and third insulationlayers are referred to in the art as an insulation stack. The coil layer416 and the first, second and third insulation layers 418, 420 and 422are sandwiched between first and second pole piece layers 424 and 426.The first and second pole piece layers 424 and 426 are magneticallycoupled at a back gap 428 and have first and second pole tips 430 and432 which are separated by a write gap layer 434 at the ABS 440. Aninsulation layer 436 is located between the second shield layer 414 andthe first pole piece layer 424. Since the second shield layer 414 andthe first pole piece layer 424 are separate layers this read/write headis known as a “piggyback” read/write head.

[0040]FIG. 5 is the same as FIG. 4 except the second shield layer 514and the first pole piece layer 524 are a common layer. This type ofread/write head is known as a “merged” head 500. The insulation layer436 of the piggyback head in FIG. 4 is omitted in the merged head 500 ofFIG. 5.

FIRST EXAMPLE

[0041]FIG. 6 shows an air bearing surface (ABS) view, not to scale, of adual magnetic tunnel junction (MTJ) sensor 600 according to a preferredembodiment of the present invention. The MTJ sensor 600 comprises endregions 604 and 606 separated from each other by a central region 602.The seed layer 614 is a layer deposited to modify the crystallographictexture or grain size of the subsequent layers, and may not be neededdepending on the subsequent layer. A first MTJ stack 608 deposited overthe seed layer 614 comprises a first antiferromagnetic (AFM1) layer 616,a first AP-pinned layer 617, an electrically insulating tunnel barrierlayer 624 and a first sense layer 625. The first AP-pinned layer 617 isformed of two ferromagnetic layers 618 and 622 separated by anantiparallel coupling (APC) layer 620. The APC layer is formed of anonmagnetic material, preferably ruthenium (Ru) that allows the twoferromagnetic layers 618 and 622 to be strongly antiparallel-coupledtogether. The AFM1 layer 616 has a thickness at which the desiredexchange properties are achieved, typically 100-300 Å.

[0042] A longitudinal bias stack 610 sequentially deposited over thefirst MTJ stack 608 comprises a first decoupling layer 629, a firstferromagnetic (FM1) layer 630, a third antiferromagnetic (AFM3) layer632, a second ferromagnetic (FM2) layer 634 and a second decouplinglayer 633. A second MTJ stack 612 deposited over the longitudinal biasstack 610 comprises a second sense layer 639, a second tunnel barrierlayer 640, a second AP-pinned layer 641 and an antiferromagnetic (AFM2)layer 648. The second AP-pinned layer 641 is formed of two ferromagneticlayers 642 and 646 separated by an antiparallel coupling (APC) layer644. The APC layer is formed of a nonmagnetic material, preferablyruthenium (Ru) that allows the two ferromagnetic layers 642 and 646 tobe strongly antiparallel-coupled together. The AFM2 layer 648 has athickness at which the desired exchange properties are achieved,typically 100-300 Å. A cap layer 650, formed on the AFM2 layer 648,completes the central region 606 of the dual SV sensor 600.

[0043] The AFM1 layer 616 is exchange-coupled to the first AP-pinnedlayer 617 to provide a pinning magnetic field to pin the magnetizationsof the two ferromagnetic layers of the first AP-pinned layerperpendicular to the ABS as indicated by an arrow tail 619 and an arrowhead 623 pointing into and out of the plane of the paper, respectively.The first sense layer 625 has a magnetization 627 that is free to rotatein the presence of an external (signal) magnetic field. Themagnetization 627 of the first sense layer 625 is preferably orientedparallel to the ABS in the absence of an external magnetic field.

[0044] The AFM2 layer 648 is exchange-coupled to the second AP-pinnedlayer 641 to provide a pinning magnetic field to pin the magnetizationsof the two ferromagnetic layers of the second AP-pinned layerperpendicular to the ABS as indicated by an arrow head 643 and an arrowtail 645 pointing out of and into the plane of the paper, respectively.The second sense layer 639 has a magnetization 637 that is free torotate in the presence of an external (signal) magnetic field. Themagnetization 637 of the second sense layer 639 is preferably orientedparallel to the ABS in the absence of an external magnetic field.

[0045] The AFM3 layer 632 is exchange-coupled to the FM1 layer 630 andthe FM2 layer 634 to provide pinning fields to pin the magnetizations631 and 635, respectively, parallel to the plane of the ABS. Themagnetizations 631 and 635 provide longitudinal bias fields which formflux closures with the first and second sense layers 625 and 639,respectively, to stabilize the first and second sense layers 625 and639.

[0046] First and second shield layers 652 and 654 adjacent to the seedlayer 614 and the cap layer 650, respectively, provide electricalconnections for the flow of a sensing current Is from a current source660 to the MTJ sensor 600. A signal detector 670 which is electricallyconnected to the first and second shield layers 652 and 654 senses thechange in resistance due to changes induced in the sense layers 625 and639 by the external magnetic field (e.g., field generated by a data bitstored on a disk). The external field acts to rotate the magnetizationsof the sense layers 625 and 639 relative to the magnetizations of thepinned layers 622 and 642 which are preferably pinned perpendicular tothe ABS. The signal detector 670 preferably comprises a partial responsemaximum likelihood (PRML) recording channel for processing the signaldetected by MTJ sensor 600. Alternatively, a peak detect channel or amaximum likelihood channel (e.g., 1,7 ML) may be used. The design andimplementation of the aforementioned channels are known to those skilledin the art. The signal detector 670 also includes other supportingcircuitries such as a preamplifier (electrically placed between thesensor and the channel) for conditioning the sensed resistance changesas is known to those skilled in the art.

[0047] The MTJ sensor 600 is fabricated in an integrated ion beam/DCmagnetron sputtering system to sequentially deposit the multilayerstructure shown in FIG. 6. The sputter deposition process is carried outin the presence of a longitudinal magnetic field of about 40 Oe. Thefirst shield layer 652 formed of Ni—Fe having a thickness of 10000 Å isdeposited on a substrate 601. The seed layer 614 is a bilayer with afirst sublayer of tantalum (Ta) having a thickness of 30 Å and a secondsublayer of Ni—Fe having a thickness of 10 Å deposited on the firstshield layer 652. The first MTJ stack 608 is formed on the seed layer bysequentially depositing the AFM1 layer 616 of Pt—Mn having a thicknessof about 160 Å, the ferromagnetic layer 618 of Co—Fe having a thicknessof about 12 Å, the APC layer 620 of ruthenium (Ru) having a thickness ofabout 8 Å, the ferromagnetic layer 622 of Co—Fe having a thickness ofabout 18 Å, the first tunnel barrier layer 624 of Al—O having athickness of about 6 Å, and the first sense layer 625 of Co—Fe having athickness of about 18 Å. The tunnel barrier layer 624 is formed bydepositing and an aluminum (Al) film with DC-magnetron sputtering from apure Al target in an argon gas of 3 mTorr, and then exposing to anoxygen gas of 2 Torr for 4 minutes. This optimum in situ oxidation isincorporated into this Al—O formation process for attaining a hightunneling magnetoresistance and low junction resistance.

[0048] The longitudinal bias stack 610 is formed on the first MTJ stack608 by sequentially depositing the first decoupling layer 629 comprisinga first sublayer 626 of Cu—O having a thickness of about 10 Å and asecond sublayer 628 of ruthenium (Ru) having a thickness of about 20 Å,the FM1 layer 630 of Co—Fe having a thickness of about 24 Å, the AFM3layer 632 of Ir—Mn having a thickness of about 60 Å, the FM2 layer 634of Co—Fe having a thickness of about 24 Å, and the second decouplinglayer 636 comprising a first sublayer 636 of ruthenium (Ru) having athickness of about 20 Å and a second sublayer 638 of Cu—O having athickness of about 10 Å. The Cu—O sublayers 626 and 638 are formed bydepositing a copper (Cu) film with DC-magnetron sputtering from a pureCu target in a mixture of argon and oxygen gases of 2.985 and 0.015mTorr, respectively, and then exposing to a mixture of argon and oxygengases of 2.94 and 0.06 mTorr, respectively, for 4 minutes. The Cu—Ofilms facilitate the sense layers to exhibit good soft magneticproperties.

[0049] The second MTJ stack 612 is formed on the longitudinal bias stack610 by sequentially depositing the second sense layer 639 of Co—Fehaving a thickness of about 18 Å, the second tunnel barrier layer 640 ofAl—O having a thickness of about 6 Å, the ferromagnetic layer 642 ofCo—Fe having a thickness of about 18 Å, the APC layer 644 of ruthenium(Ru) having a thickness of about 8 Å, the ferromagnetic layer 646 ofCo—Fe having a thickness of about 12 Å, and the AFM2 layer 648 of Pt—Mnhaving a thickness of about 160 A. The tunnel barrier layer 640 isformed by depositing and an aluminum (Al) film with DC-magnetronsputtering from a pure Al target in an argon gas of 3 mTorr, and thenexposing to an oxygen gas of 2 Torr for 4 minutes. This optimum in situoxidation is incorporated into this Al—O formation process for attaininga high tunneling magnetoresistance and low junction resistance. The caplayer 650 is a bilayer with a first sublayer of ruthenium (Ru) having athickness of 40 Å and a second sublayer of tantalum (Ta) having athickness of 30 Å formed over the AFM2 layer 648.

[0050] The second shield layer 654 formed of Ni—Fe having a thickness of10000 Å is deposited over the cap layer 650. An insulating layer 656formed of Al₂O₃ deposited between the first shield layer 652 and thesecond shield layer 654 provides electrical insulation between theshields/leads and prevents shunting of the sense current around theactive region 602 of the dual MTJ sensor 600.

[0051] After the deposition of the central portion 602 is completed, thesensor is annealed for 2 hours at 280° C. in the presence of a magneticfield of about 10,000 Oe in a transverse direction perpendicular to theABS and is then cooled while still in the magnetic field to set theexchange coupling of the AFM1 and AFM2 layers 616 and 648 with theAP-pinned layers 617 and 641, respectively, so that the magnetizationsin the two AP-pinned layers are perpendicular to the ABS with netmagnetic moments canceling each other. This results in cancellation ofdemagnetization fields between the the AP-pinned layers 617 and 641.

[0052] After the first anneal, a second anneal is carried out for 2hours at 240° C. in the presence of a magnetic field of 200 Oe in alongitudinal direction parallel to the ABS. Because the blockingtemperature of the Pt—Mn antiferromagnetic material (>360° C.) of theAFM1 and AFM2 layers is higher than 240° C., the magnetizations of thefirst and second AP-pinned layers 617 and 641 are not rotated while themagnetizations 631 and 635 in the longitudinal bias stack are orientedin the longitudinal direction due to the lower (less than 240° C.)blocking temperature of the Ir—Mn antiferromagnetic material of the AFM3layer. After the second anneal, the magnetization 631 of the FM1 layer630 forms a flux closure with the magnetization 627 of the first senselayer 625 providing stability for the first sense layer 625. Similarly,the magnetization 635 of the FM2 layer 634 forms a flux closure with themagnetization 637 of the second sense layer 639 providing stability ofthe second sense layer 639.

[0053] The Cu—O/Ru and Ru/Cu—O films are used as first and seconddecoupling layers 629 and 633. Either one of the Cu—O or Ru films arenot used alone as a decoupling layer since strong exchange couplingoccurs across either film and full decoupling can only be attained whenthe film thickness is far greater than 30 Å. In order to attain strongmagnetostatic interactions from the flux closures, the decoupling layerthickness is preferred to be as small as possible, but not to be toosmall to induce the echange coupling. The Cu—O film of the decouplinglayers is adjacent to the Co—Fe sense layers to promote good softmagnetic properties. The Ru film of the decoupling layers is also usedas a seed layer for the Co—Fe/Ir—Mn/Co—Fe longitudinal bias layers topromote high unidirectional anisotropy fields (H_(UA)).

[0054] In this preferred embodiment, the Co—Fe/Ir—Mn/Co—Fe film stack isused for antiferromagnetic stabilization of the dual MTJ sensors.Alternatively, a Ni—Fe(10 Å)/Co—Pt(40 Å)/Ni—Fe(10 Å) film stack can beused to replace the Co—Fe/Ir—Mn/Co—Fe film stack for hard magneticstabilization. The Ni—Fe film is adjacent to the Co—Pt film in order toreduce stray fields from the Co—Pt film and improve its squareness. Inthis alternate embodiment, the longitudinal field anneal used tolongitudinally orient the magnetizations of the Co—Fe/Ir—Mn/Co—Fe stackis eliminated, but magnetic setting of the Ni—fe/Co—Pt/Ni—Fe stack in afield of 3 kOe must be conducted at room temperature after the headfabrication process.

SECOND EXAMPLE

[0055]FIG. 7 shows an air bearing surface (ABS) view, not to scale, of adual magnetic tunnel junction (MTJ) sensor 700 according to anotherembodiment of the present invention. The dual MTJ sensor 700 is the sameas the dual MTJ sensor 600 shown in FIG. 6 except that in order toachieve a read gap thickness of 50 nm the AFM1 and AFM2 layers 616 and648 have been eliminated and the AFM3 layer 632 of the longitudinal biasstack 610 has been replaced by an AFM3 layer 716 of Pt—Mn having athickness of 160 Å. The MTJ sensor 700 comprises a first MTJ stack 708,a second MTJ stack 712 and a longitudinal bias stack 710 disposedbetween the first and second MTJ stacks 708 and 712. In this embodimentthe first MTJ stack 708 is the same as MTJ stack 608 without the AFM1layer 616 and the second MTJ stack 712 is the same as MTJ stack 612without the AFM2 layer 648. The longitudinal bias stack 710 is the sameas bias stack 610 with the AFM3 layer of Ir—Mn replaced with an AFM3layer 716 of Pt—Mn having a thickness of 160 Å.

[0056] The MTJ sensor 700 is fabricated in an integrated ion beam/DCmagnetron sputtering system to sequentially deposit the multilayerstructure shown in FIG. 7. The deposition process is the same as theprocess used to fabricate the MTJ sensor 600.

[0057] After the deposition of the central portion 602 is completed, theMTJ sensor 700 is annealed for 2 hours at 280° C. in the presence of amagnetic field of 200 Oe in a longitudinal direction parallel to theABS. The magnetic field is higher than the uniaxial anisotropy fieldH_(K) (≦20 Oe) of the as-deposited Co—Fe/Pt—Mn/Co—Fe films so thatstrong exchange coupling in the Co—Fe/Pt—Mn/Co—Fe films can be developedin a longitudinal direction parallel to the ABS during annealing. Themagnetic field is less than the spin flop field H_(SF) (≧1 kOe) of thefirst and second pinned layers 617 and 641 so that strong antiparallelcoupling across the Ru APC layers in the first and second AP-pinnedlayers 617 and 641 is not interrupted during annealing.

[0058] Although the Pt—Mn AFM layers are not used in the first andsecond MTJ stacks 708 and 712 to produce H_(UA) for transverse pinning,the transverse pinning can still be attained due to a strong spin-flopfield (H_(SP)) induced from antiparallel coupling across the Ru APClayers. The transverse pinning can be further reinforced if the Co—Fefilms adjacent to the Ru APC layers have a high intrinsic uniaxialanisotropy field (H_(K)) and a high positive saturation magnetostriction(λ_(S)). The high λ_(S) is needed to stress-induce a high extrinsicuniaxial anisotropy field (H_(K)′), determined fromH_(K)′=3(λ_(S)/M_(S)) σ after sensor lapping. The Co₉₀—Fe₁₀ (in atomic%) commonly used for the ferromagnetic layers of the AP-pinned layers617 and 641 has an H_(K) of 16 Oe. When the Fe content is increased to20 at. %, H_(K) becomes 30 Oe and the λ_(S) increases to 35.1×10⁻⁶(corresponding to 142 Oe). Hence, in this alternative embodiment, aCo—Fe film with an Fe content of 20 at. % or higher (up to 50 at. %) ispreferably used for the ferromagnetic layers of the AP-pinned layers.

[0059] Although the total uniaxial anisotropy field H_(K)+H_(K)′ (172.5Oe) is not as high as H_(UA) (600 Oe) and H_(SP) (900 Oe), it has twomajor unique features, leading it to play an important role in providingtransverse pinning. First, H_(K)+H_(K)′ is determined only from theCo—Fe film itself, while H_(UA) and H_(SP) are determined not only fromthe Co—Fe film, but also from its adjacent Ru and Pt—Mn films. As aresult, the temperature dependence of H_(K)+H_(K)′ is determined by theCurie temperature of the Co—Fe film (˜700° C.), while the temperaturedependence of H_(UA) is determined by the blocking temperature of theexchange-coupled Pt—Mn/Co—Fe films (˜360° C.), and the temperaturedependence of H_(SP) is determined by the critical temperature of theantiparallel-coupled Co—Fe/Ru/Co—Fe films. Since the Curie temperatureis much higher than the blocking and critical temperatures, H_(K)+H_(K)′can remain nearly unchanged at elevated sensor operation temperatures(˜180° C.). Thus H_(K)+H_(K)′ may play a crucial role in improvingthermal stability. Second, H_(K)+H_(K)′ substantially reduces edgecurling effects of magnetizations of the ferromagnetic AP-pinned layers.The reduction in the edge curling effects results in more uniformmagnetization along the sensor height, therefore providing better fluxclosure to cancel magnetization more efficiently. Hence, even withoutthe use of the Pt—Mn film for transverse pinning, the transverse pinningfield resulting from H_(SP), H_(K) and H_(K)′ should be high enough forproper sensor operation.

[0060] While the present invention has been particularly shown anddescribed with reference to the preferred embodiments, it will beunderstood by those skilled in the art that various changes in form anddetail may be made without departing from the spirit, scope and teachingof the invention. Accordingly, the disclosed invention is to beconsidered merely as illustrative and limited only as specified in theappended claims.

We claim:
 1. A dual magnetic tunnel junction (MTJ) sensor, comprising: afirst magnetic tunnel junction (MTJ) stack; a second magnetic tunneljunction (MTJ) stack; and a longitudinal bias stack disposed betweensaid first and second MTJ stacks.
 2. The dual MTJ sensor as recited inclaim 1, wherein said longitudinal bias stack comprises: a firstferromagnetic (FM1) layer; a second ferromagnetic (FM2) layer; anantiferromagnetic layer disposed between said FM1 and FM2 layers; afirst decoupling layer disposed between said first MTJ stack and saidFM1 layer; and a second decoupling layer disposed between said FM2 layerand said second MTJ stack.
 3. A dual magnetic tunnel junction (MTJ)sensor, comprising: a first magnetic tunnel junction (MTJ) stack,comprising: a first antiferromagnetic (AFM1) layer; a first antiparallel(AP)-pinned layer in contact with said AFM1 layer; a first sense layerof ferromagnetic material; a tunnel barrier layer disposed between saidfirst sense layer and said first AP-pinned layer; a second magnetictunnel junction (MTJ) stack, comprising: a second antiferromagnetic(AFM2) layer; a second antiparallel (AP)-pinned layer in contact withsaid AFM2 layer; a second sense layer of ferromagnetic material; atunnel barrier layer disposed between said second sense layer and saidsecond AP-pinned layer; and a longitudinal bias stack disposed betweensaid first and second sense layers, said longitudinal bias stackcomprising: a first ferromagnetic (FM1) layer; a second ferromagnetic(FM2) layer; a third antiferromagnetic (AFM3) layer disposed betweensaid FM1 and FM2 layers; a first decoupling layer disposed between saidfirst sense layer and said FM1 layer; and a second decoupling layerdisposed between said FM2 layer and said second sense layer.
 4. The dualMTJ sensor as recited in claim 3, wherein said AFM1 and AFM2 layers aremade of Pt—Mn.
 5. The dual MTJ sensor as recited in claim 3, whereinsaid AFM3 layer is made of Ir—Mn.
 6. The dual MTJ sensor as recited inclaim 3, wherein a first blocking temperature of the AFM1 and AFM2layers is greater than a second blocking temperature of the AFM3 layer.7. The dual MTJ sensor as recited in claim 3, wherein said firstdecoupling layer comprises: a first sublayer made of Cu—O adjacent tosaid first sense layer; and a second sublayer made of ruthenium disposedbetween said first sublayer and said FM1 layer.
 8. The dual MTJ sensoras recited in claim 3, wherein said second decoupling layer comprises: asecond sublayer made of Cu—O adjacent to said second sense layer; and afirst sublayer made of ruthenium (Ru) disposed between said secondsublayer and said FM2 layer.
 9. A dual magnetic tunnel junction (MTJ)sensor, comprising: a first magnetic tunnel junction (MTJ) means forproviding a first readback signal in response to a magnetic signalfield, said first MTJ means including a first sense layer meansresponsive to said magnetic signal field; a second magnetic tunneljunction (MTJ) means for providing a second readback signal in responseto a magnetic signal field, said second MTJ means including a secondsense layer means responsive to said magnetic signal field; and a biasmeans for providing longitudinal bias fields at said first and secondsense layer means to stabilize said first and second MTJ means, saidbias means disposed between said first and second sense layer means. 10.A magnetic read/write head, comprising: a write head including: at leastone coil layer and an insulation stack, the coil layer being embedded inthe insulation stack; first and second pole piece layers connected at aback gap and having pole tips with edges forming a portion of an airbearing surface (ABS); the insulation stack being sandwiched between thefirst and second pole piece layers; and a write gap layer sandwichedbetween the pole tips of the first and second pole piece layers andforming a portion of the ABS; a read head including: a dual magnetictunnel junction (MTJ) sensor, the dual MTJ sensor being sandwichedbetween first and second shield layers, the dual MTJ sensor comprising:a first magnetic tunnel junction (MTJ) stack; a second magnetic tunneljunction (MTJ) stack; and a longitudinal bias stack disposed betweensaid first and second MTJ stacks; and an insulation layer disposedbetween the second shield layer of the read head and the first polepiece layer of the write head.
 11. The magnetic read/write head asrecited in claim 10, wherein said longitudinal bias stack comprises: afirst ferromagnetic (FM1) layer; a second ferromagnetic (FM2) layer; anantiferromagnetic layer disposed between said FM1 and FM2 layers; and afirst decoupling layer disposed between said first MTJ stack and saidFM1 layer; and a second decoupling layer disposed between said FM2 layerand said second MTJ stack.
 12. A magnetic read/write head, comprising: awrite head including: at least one coil layer and an insulation stack,the coil layer being embedded in the insulation stack; first and secondpole piece layers connected at a back gap and having pole tips withedges forming a portion of an air bearing surface (ABS); the insulationstack being sandwiched between the first and second pole piece layers;and a write gap layer sandwiched between the pole tips of the first andsecond pole piece layers and forming a portion of the ABS; a read headincluding: a dual magnetic tunnel junction (MTJ) sensor, the MTJ sensorbeing sandwiched between first and second shield layers, the MTJ sensorcomprising: a first magnetic tunnel junction (MTJ) stack, comprising: afirst antiferromagnetic (AFM1) layer; a first antiparallel (AP)-pinnedlayer in contact with said AFM1 layer; a first sense layer offerromagnetic material; a tunnel barrier layer disposed between saidfirst sense layer and said first AP-pinned layer; a second magnetictunnel junction (MTJ) stack, comprising: a second antiferromagnetic(AFM2) layer; a second antiparallel (AP)-pinned layer in contact withsaid AFM2 layer; a second sense layer of ferromagnetic material; atunnel barrier layer disposed between said second sense layer and saidsecond AP-pinned layer; and a longitudinal bias stack disposed betweensaid first and second sense layers, said longitudinal bias stackcomprising: a first ferromagnetic (FM1) layer; a second ferromagnetic(FM2) layer; a third antiferromagnetic (AFM3) layer disposed betweensaid FM1 and FM2 layers; a first decoupling layer disposed between saidfirst sense layer and said FM1 layer; and a second decoupling layerdisposed between said FM2 layer and said second sense layer; and aninsulation layer disposed between the second shield layer of the readhead and the first pole piece layer of the write head.
 13. The magneticread/write head as recited in claim 12, wherein said AFM1 and AFM2layers are made of Pt—Mn.
 14. The magnetic read/write head as recited inclaim 12, wherein said AFM3 layer is made of Ir—Mn.
 15. The magneticread/write head as recited in claim 12 wherein a first blockingtemperature of the AFM1 and AFM2 layers is greater than a secondblocking temperature of the AFM3 layer.
 16. A magnetic read/write head,comprising: a write head including: at least one coil layer and aninsulation stack, the coil layer being embedded in the insulation stack;first and second pole piece layers connected at a back gap and havingpole tips with edges forming a portion of an air bearing surface (ABS);the insulation stack being sandwiched between the first and second polepiece layers; and a write gap layer sandwiched between the pole tips ofthe first and second pole piece layers and forming a portion of the ABS;a read head including: a dual magnetic tunnel junction (MTJ) sensor, theMTJ sensor being sandwiched between first and second shield layers, theMTJ sensor comprising: a first magnetic tunnel junction (MTJ) means forproviding a first readback signal in response to a magnetic signalfield, said first MTJ means including a first sense layer meansresponsive to said magnetic signal field; a second magnetic tunneljunction (MTJ) means for providing a second readback signal in responseto a magnetic signal field, said second MTJ means including a secondsense layer means responsive to said magnetic signal field; and a biasmeans for providing longitudinal bias fields at said first and secondsense layer means to stabilize said first and second MTJ means, saidbias means disposed between said first and second sense layer means; andan insulation layer disposed between the second shield layer of the readhead and the first pole piece layer of the write head.
 17. A disk drivesystem comprising: a magnetic recording disk; a magnetic read/write headfor magnetically recording data on the magnetic recording disk and forsensing magnetically recorded data on the magnetic recording disk, saidmagnetic read/write head comprising: a write head including: at leastone coil layer and an insulation stack, the coil layer being embedded inthe insulation stack; first and second pole piece layers connected at aback gap and having pole tips with edges forming a portion of an airbearing surface (ABS); the insulation stack being sandwiched between thefirst and second pole piece layers; and a write gap layer sandwichedbetween the pole tips of the first and second pole piece layers andforming a portion of the ABS; a read head including: a dual magnetictunnel junction (MTJ) sensor, the MTJ sensor being sandwiched betweenfirst and second shield layers, the MTJ sensor comprising: a firstmagnetic tunnel junction (MTJ) stack; a second magnetic tunnel junction(MTJ) stack; and a longitudinal bias stack disposed between said firstand second MTJ stacks; and an insulation layer disposed between thesecond shield layer of the read head and the first pole piece layer ofthe write head; an actuator for moving said magnetic read/write headacross the magnetic disk so that the read/write head may accessdifferent regions of the magnetic recording disk; and a recordingchannel coupled electrically to the write head for magneticallyrecording data on the magnetic recording disk and to the MTJ sensor ofthe read head for detecting changes in resistance of the MTJ sensor inresponse to magnetic fields from the magnetically recorded data.
 18. Thedisk drive system as recited in claim 17, wherein said longitudinal biasstack comprises: a first ferromagnetic (FM1) layer; a secondferromagnetic (FM2) layer; an antiferromagnetic layer disposed betweensaid FM1 and FM2 layers; a first decoupling layer disposed between saidfirst MTJ stack and said FM1 layer; and a second decoupling layerdisposed between said FM2 layer and said second MTJ stack.
 19. A diskdrive system comprising: a magnetic recording disk; a magneticread/write head for magnetically recording data on the magneticrecording disk and for sensing magnetically recorded data on themagnetic recording disk, said magnetic read/write head comprising: awrite head including: at least one coil layer and an insulation stack,the coil layer being embedded in the insulation stack; first and secondpole piece layers connected at a back gap and having pole tips withedges forming a portion of an air bearing surface (ABS); the insulationstack being sandwiched between the first and second pole piece layers;and a write gap layer sandwiched between the pole tips of the first andsecond pole piece layers and forming a portion of the ABS; a read headincluding: a dual magnetic tunnel junction (MTJ) sensor, the MTJ sensorbeing sandwiched between first and second shield layers, the MTJ sensorcomprising: a first magnetic tunnel junction (MTJ) stack, comprising: afirst antiferromagnetic (AFM1) layer; a first antiparallel (AP)-pinnedlayer in contact with said AFM1 layer; a first sense layer offerromagnetic material; a tunnel barrier layer disposed between saidfirst sense layer and said first AP-pinned layer; a second magnetictunnel junction (MTJ) stack, comprising: a second antiferromagnetic(AFM2) layer; a second antiparallel (AP)-pinned layer in contact withsaid AFM2 layer; a second sense layer of ferromagnetic material; atunnel barrier layer disposed between said second sense layer and saidsecond AP-pinned layer; and a longitudinal bias stack disposed betweensaid first and second sense layers, said longitudinal bias stackcomprising: a first ferromagnetic (FM1) layer; a second ferromagnetic(FM2) layer; a third antiferromagnetic (AFM3) layer disposed betweensaid FM1 and FM2 layers; a first decoupling layer disposed between saidfirst sense layer and said FM1 layer; and a second decoupling layerdisposed between said FM2 layer and said second sense layer; and aninsulation layer disposed between the second shield layer of the readhead and the first pole piece layer of the write head; and an actuatorfor moving said magnetic read/write head across the magnetic disk sothat the read/write head may access different regions of the magneticrecording disk; and a recording channel coupled electrically to thewrite head for magnetically recording data on the magnetic recordingdisk and to the MTJ sensor of the read head for detecting changes inresistance of the MTJ sensor in response to magnetic fields from themagnetically recorded data.
 20. The disk drive system as recited inclaim 19, wherein said AFM1 and AFM2 layers are made of Pt—Mn.
 21. Thedisk drive system as recited in claim 19, wherein said AFM3 layer ismade of Ir—Mn.
 22. The disk drive system as recited in claim 19, whereina first blocking temperature of the AFM1 and AFM2 layers is greater thana second blocking temperature of the AFM3 layer.
 23. A disk drive systemcomprising: a magnetic recording disk; a magnetic read/write head formagnetically recording data on the magnetic recording disk and forsensing magnetically recorded data on the magnetic recording disk, saidmagnetic read/write head comprising: a write head including: at leastone coil layer and an insulation stack, the coil layer being embedded inthe insulation stack; first and second pole piece layers connected at aback gap and having pole tips with edges forming a portion of an airbearing surface (ABS); the insulation stack being sandwiched between thefirst and second pole piece layers; and a write gap layer sandwichedbetween the pole tips of the first and second pole piece layers andforming a portion of the ABS; read head including: a dual magnetictunnel junction (MTJ) sensor, the MTJ sensor being sandwiched betweenfirst and second shield layers, the MTJ sensor comprising: a firstmagnetic tunnel junction (MTJ) means for providing a first readbacksignal in response to a magnetic signal field, said first MTJ meansincluding a first sense layer means responsive to said magnetic signalfield; a second magnetic tunnel junction (MTJ) means for providing asecond readback signal in response to a magnetic signal field, saidsecond MTJ means including a second sense layer means responsive to saidmagnetic signal field; and a bias means for providing longitudinal biasfields at said first and second sense layer means to stabilize saidfirst and second MTJ means, said bias means disposed between said firstand second sense layer means; an insulation layer disposed between thesecond shield layer of the read head and the first pole piece layer ofthe write head; and an actuator for moving said magnetic read/write headacross the magnetic disk so that the read/write head may accessdifferent regions of the magnetic recording disk; and a recordingchannel coupled electrically to the write head for magneticallyrecording data on the magnetic recording disk and to the MTJ sensor ofthe read head for detecting changes in resistance of the MTJ sensor inresponse to magnetic fields from the magnetically recorded data.
 24. Amethod of fabricating a dual magnetic tunnel junction (MTJ) sensor whichcomprises the steps of: a) sputter depositing the multilayer dual MTJsensor including a first magnetic tunnel junction (MTJ) stack, a secondmagnetic tunnel junction (MTJ) stack and a longitudinal bias stackdisposed between the first and second MTJ stacks; b) annealing the dualMTJ sensor at a first temperature in a first magnetic field oriented ina transverse direction perpendicular to an air bearing surface; and c)annealing the dual MTJ sensor at a second temperature in a secondmagnetic field oriented in a longitudinal direction parallel to said airbearing surface, wherein said second temperature is less than said firsttemperature and said second magnetic field has a magnitude smaller thansaid first magnetic field.
 25. The method of fabricating a dual MTJsensor as recited in claim 24, wherein said first temperature is about280° C. and said second temperature is about 240° C.
 26. The method offabricating a dual MTJ sensor as recited in claim 24, wherein said firstfirst magnetic field has a magnitude of about 10,000 Oe and said secondmagnetic field has a magnitude of about 200 Oe.